1. Field of the Invention
The present invention relates to the field of semiconductor devices, and to processes requiring the formation of ohmic contacts at relatively low temperatures. More particularly, the invention relates to fabrication processes and devices such as multijunction solar cells based on III-V semiconductor compounds including a metamorphic layer. Such devices are also known as inverted metamorphic multijunction solar cells.
2. Description of the Related Art
Photovoltaic cells, also called solar cells, are one of the most important new energy sources that have become available in the past several years. Considerable effort has gone into solar cell development. As a result, solar cells are currently being used in a number of commercial and consumer-oriented applications. While significant progress has been made in this area, the requirement for solar cells to meet the needs of more sophisticated applications has not kept pace with demand. Applications such as concentrator terrestrial power systems and satellites used in data communications have dramatically increased the demand for solar cells with improved power and energy conversion characteristics.
In satellite and other space related applications, the size, mass and cost of a satellite power system are dependent on the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of power provided. Thus, as the payloads become more sophisticated, solar cells, which act as the power conversion devices for the on-board power systems, become increasingly more important.
Solar cells are often fabricated in vertical, multijunction structures, and disposed in horizontal arrays, with the individual solar cells connected together in a series. The shape and structure of an array, as well as the number of cells it contains, are determined in part by the desired output voltage and current.
Inverted metamorphic solar cell structures such as described in M. W. Wanlass et al., Lattice Mismatched Approaches for High Performance, III-V Photovoltaic Energy Converters (Conference Proceedings of the 31st IEEE Photovoltaic Specialists Conference, Jan. 3-7, 2005, IEEE Press, 2005) present an important conceptual starting point for the development of future commercial high efficiency solar cells. The structures described in such reference present a number of practical difficulties relating to the appropriate choice of materials and fabrication steps, for a number of different layers of the cell.
Prior to the present invention, the materials and fabrication steps disclosed in the prior art have not been adequate to produce a commercially viable and energy efficient solar cell using commercially established fabrication processes for producing an inverted metamorphic multijunction cell structure.